Atrial fibrillation is probably the most common cardiac arrhythmia Although it is not usually a life threatening arrhythmia, it is associated with strokes thought to be caused by blood clots forming in areas of stagnant blood flow as a result of prolonged atrial fibrillation. In addition, patients afflicted with atrial fibrillation generally experience palpitations of the heart and may even experience dizziness or even loss of consciousness.
As used herein, "atrial fibrillation" (AF) will be used to ream atrial fibrillation, flutter, or tachycardia correctable by defibrillation, low energy cardioversion, antitachycardia pacing (ATP), or drugs. "Cardioversion" will refer to any of these means.
Atral fibrillation occurs suddenly and many times can only be corrected by a discharge of electrical energy to the heart through the skin of the patient by way of an external defibrillator of the type well known in the art. This treatment is commonly referred to as synchronized cardioversion and, as its name implies, involves applying electrical defibrillating energy to the heart in synchronism with a detected ventricular electrical activation (R wave) of the heart. The treatment is very painful and, unfortunately, often only results in temporary relief for patients, lasting but a few weeks.
Drugs are available for reducing the incidence of atrial fibrillation. However, these drugs have many side effects and many patients are resistant to them which greatly reduces their therapeutic effect.
Implantable atrial defibrillators have been proposed to provide patients suffering from occurrences of atrial fibrillation with relief. However, physicians have been reluctant to implant such devices for fear that the therapy for atrial fibrillation could produce potentially lethal ventricular arrhythmias. In view of this, developers of implantable ventricular defibrillators have begun to develop dual chamber implantable defibrillators for those patients who suffer from atrial and ventricular cardiac arrhythmias.
Two proposed atrial defibrillators, although represented as being implantable, were not fully automatic, requiring human interaction for cardioverting or defibrillating the heart. Both are disclosed in U.S. Pat. No. 3,952,750 to Mirowski et al., which patent is incorporated herein by reference. Both of these proposed defibrillators require the patient to recognize the symptoms of atrial fibrillation with one defibrillator requiring a visit to a physician to activate the defibrillator and the other defibrillator requiring the patient to activate the defibrillator with an external magnet.
An automatic implantable atrial defibrillator which provides a warning to the patient prior to delivery of cardioversion therapy is disclosed in U.S. Pat. No. 5,332,400 to Alferness, which patent is incorporated herein by reference. The atrial defibrillator disclosed in Alferness includes an atrial fibrillation detector which, responsive to sensed atrial activity, determines when the atria of the heart are in need of cardioversion. When the atrial fibrillation detector determines that the atria are in fibrillation and thus in need of cardioversion, the atrial fibrillation detector causes a cardioverter stage to deliver defibrillating or cardioverting electrical energy to the atria in timed relation to a detected ventricular electrical activation (R wave) of the heart.
Unfortunately, the quantity of electrical energy which is required to cardiovert or defibrillate the atria is sufficient, in most cases, to cause a sudden pain in the patient's chest area or stun the patient. In addition, the successful cardioversion or defibrillation of the atria may also result in a rapid decrease in the patient's heart rate from a high and possibly variable heart rate. This rapid change in heart rate can, for some patients, cause discomfort or even temporary dizziness. As a result, it is highly desirable as described in the Alferness patent to provide a warning to a patient prior to the delivery of cardioverting or defibrillating electrical energy to the patient's atria. The warning is in the form of electrical energy applied to internal tissue of the patient and being of a quantity so as to be discernible by the patient without pain or other undesirable effects.
One problem with the system of Alferness is that the cardioverting energy is automatically delivered to the patient a prescribed period of time following the warning signal. Since atrial fibrillation is fairly well tolerated by most patients, it would be desirable to allow for a controllable delay in the delivery of the cardioverting energy to allow the patient to better control where he is when cardioversion occurs. Thus, while the atrial defibrillator of Alferness would warn a patient of an impending shock and allow the patient to pull over to the side of the road when driving, it would be desirable to allow patients to delay therapy for long enough for them to reach their destination and/or a safer, more comfortable location.
One problem with allowing the patient to delay delivery of therapy for atrial defibrillation is that, as mentioned above, blood clots may form in areas of stagnant blood flow as a result of prolonged atrial fibrillation. It is thus generally agreed that internal electrical cardioversion of atrial fibrillation should not be attempted without the use of anticoagulants if the patient has been in atrial fibrillation for more than 48 hours, and anticoagulation may be desirable even earlier, such as after 24 hours. It is therefore desirable to allow patient control over the timing of energy delivery for atrial defibrillation but to prevent delivery beyond a time when it is risky to deliver the therapy without the administration of anticoagulants.